Effect of using different concentrations of magnesium in the water of cultivation of Pacific white shrimp, Litopenaeus vannamei (Boone, 1931) at low salinity with biofloc technology
The bioflocs system has the advantages of low water use and effluent generation, in addition to the possibility of development in low salinity. Combined with the use of artificial salinization, the viability and sustainability of aquaculture far from the coast has been increasing. Magnesium, an essential mineral required by crustaceans for growth and a more expensive constituent can be reduced in artificial salting. In order to evaluate the effect of different concentrations of magnesium in the culture water, in L. vannamei cultivated in artificially saline waters in BFT system, 6 treatments were designed, with 3 repetitions, 5 salinity in two phases following the same configuration, nursery with stocking density of 1500cam/m3, magnesium was manipulated in the Calcium - Magnesium - Potassium ratio. Control Treatment - TC (diluted sea water); TI = 1– 3 - 0.9; TII = 1 - 2 - 0.9; TIII = 1 - 1 - 0.9; TIV = 1 - 0.5 - 0.9; TV = 1 - 0.1 - 0.9. Eighteen experimental units with 40L were used for 42 days. In fattening, 500 cam/m3 reusing water with bioflocs matured for 35 days. The zootechnical performance was evaluated and an estimate of the costs of artificial salinization was made. The best results were observed in T3 (survival = 88.33 ± 10.41; feed conversion 1.8 g ± 0.07; final weight 4.63 g ± 0.53), allowing the reduction of this ion without prejudice to the prawns . Despite the feasibility of reducing the amount of salts in artificial salinization, there is still a gap in relation to the toxicity of nitrogen compounds at low salinities. Nitrate, for example, can be lethal in high concentrations and is directly related to salinity.Acute toxicity tests allow analyzing the relationship between the compound and other abiotic or biotic variables. The objective of the second study was to determine the median lethal concentration (CL50-96h) and the nitrate safety level (N-NO3 -260) in juveniles of L. vannamei at salinities 5 and 10. For salinity 5, a control and five treatments were tested, with concentrations of 100, 500, 1500, 2500 and 3500 mg.L-1. For salinity of 10 gL-1, a concentration of 4500 mg.L-1 was added. 45 juveniles per treatment were exposed to concentrations for 24, 48, 72, 96 hours. The Median Lethal Concentration (LC50) was calculated and the recommended safety level for the cultivation of L. vannamei is 60.05 and 127.61 mg.L-1 of nitrate in salinities 5 and 10 gL-1, respectively.